Frequency stabilization method of semiconductor laser and frequency-stabilized light source

Abstract: A frequency stabilization method of a semiconductor laser is provided. A driving current, a forward voltage and an output light power of the laser mounted on a heat sink is detected. A temperature of the heat sink is also detected. A consumption power of the laser is obtained from the driving current and voltage thus detected, providing a relationship between the output light power and the consumption power. The driving current is controlled so that the output light power is kept constant, and the temperature of the heat sink is controlled based on the relationship so that a temperature of an active layer of the laser is maintained. The output light power is kept constant and at the same time, any temperature change of the active layer is cancelled through the temperature control of the heat sink. Even if the consumption power changes due to an leakage current and/or a recombination current without luminescence to maintain the output light power during long time operation, the temperature of the active layer is maintained by cancelling the consumption power change through the temperature control of the heat sink. Thus, the oscillation frequency of the semiconductor laser can be stabilized at a given value.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and more particularly, to a frequency stabilization method of a semiconductor laser and a frequency-stabilized light source. 2. Description of the prior art A conventional frequency stabilization method of a semiconductor laser is disclosed in the IEEE Journal of Quantum Electronics, Vol. 28, No. 1, Page 75, January 1992, in which an oscillation frequency of the laser is stabilized at an absorption line frequency of acetylene. Fig. 1 shows a functional block diagram of the conventional frequency stabilization method. In Fig. 1, the ambient temperature of a semiconductor laser 701 is kept constant by a temperature controller 702. A DC power supply 709 supplies a driving current to the laser 701. The driving current is slightly modulated in frequency by a modulation signal outputted from an oscillator 703, resulting in frequency-modulated output light beams of the laser 701. One of the output light beams of the laser 701 is emitted from a side face of the laser 701 and used for a given application. The other of the output light beams of the laser 701 is emitted from its opposite side face and goes through an optical lens system 708 to be injected into an acetylene (C 2 H 2 ) gas cell 704. The light beam transmitted through the acetylene gas cell 704 is detected by a photodetector 705 to produce an electrical output signal. The electrical output signal is inputted into an lock-in amplifier 706 to be detected synchronously with the modulation frequency from the oscillator 703. The lock-in amplifier 706 produces an electrical output signal proportional to a difference or error between the oscillation frequency of the laser 701 and one of absorption peak frequencies of acetylene in the cell 704. The electrical output signal from the amplifier 706 is fed-back to the driving current through a PID controller 707 in which a Proportional, Integral and Differential (PID) controlling method is employed. Thus, the laser 701 is controlled so that its oscillation frequency is kept to be in accordance with the absorption peak frequency of acetylene. Due to high stability in the absorption peak frequency, the oscillation frequency of the semiconductor laser 701 can be highly stabilized or locked. With the conventional frequency stabilization method shown in Fig. 1, to obtain the difference or error between the oscillation frequency and the absorption peak frequency, the gas cell 704 and the lock-in amplifier 706 are required, and the driving current is modulated in frequency to be injected into the laser 701. As a result, there arises disadvantages that large-sized and expensive setups or apparatuses are necessary for carrying out the method and no unmodulated output light beam can be obtained. In the case of stabilizing the optical output power of the laser 701 during operation, there is another disadvantage that another photodetector is necessary in addition to the photodetector 705. Further, there is still another disadvantage that stabilizable oscillation frequencies are restricted to the absorption peak frequencies of the gas in the cell 704, so that any or arbitrary oscillation frequencies cannot be selected. Another conventional frequency stabilization method of a semiconductor laser is disclosed in the Japanese Non-Examined Patent Publication No. 64-74780, March 1989, in which a semiconductor laser temperature is detected from a forward voltage of the laser to keep the temperature constant. Fig. 2 shows a functional block diagram of the conventional frequency stabilization method. In Fig. 2, a semiconductor laser 803 and a Peltier element 802 which generates and absorbs heat are arranged in a thermostatic bath 801. The laser 803 is driven by a constant current supplied from a DC current source 804. A differential amplifier 805 detects between input terminals or electrodes of the laser 803 its forward voltage drop V f , and sends an electrical output signal proportional the voltage drop V f to a temperature controller 806. In response to the output signal from the amplifier 805, the controller 806 increases or decreases a driving current for the Peltier element 801 to thereby keep the temperature of the laser 803 constant. In general, the forward voltage drop V f of the semiconductor laser 803 is expressed as V f = k·I n (1 + I f I 0 ) e·T where I 0 is the forward saturation current, I f is a driving or exciting current, T is the absolute temperature of the laser 803, e is the charge of an electron and k is the Boltzmann's constant. To be seen from the equation (1), the forward voltage drop V f is inversely proportional to the absolute temperature T. Thus, the absolute temperature T of the laser 803 can be exactly measured from the voltage drop V f . The differential amplifier 805 produces an output signal relating to the absolute temperature T from the detected voltage drop V f and sends the signal to the temperature controller 806. In response to the signal, the controller 806 controls to keep the temperature of the laser 803 constant. The oscillation frequency of the semiconductor laser 803 is decided by the absolute temperature T and driving current I f , so that it can be seen that the oscillation frequency is stabilized if both of them are kept constant. With the another conventional frequency stabilization method shown in Fig. 2, an error tends to arise in detection of the absolute temperature T through the differential amplifier 805 because the laser 803 has an leakage current and a recombination current without luminescence both of which increase with the passage of time, providing fluctuation or deviation in the absolute temperature T and driving current I f . As a result, there arises a disadvantage that the output light power and oscillation frequency of the laser 803 deviate from the given values, respectively. In addition to the above methods, still another conventional frequency stabilization method of a semiconductor laser is disclosed in the Japanese Non-Examined Patent Publication No. 1-238083, September 1989. In the method, similar to the conventional method shown in Fig. 2, absorption peak frequencies of a gas is used. SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency stabilization method of a semiconductor laser and a frequency-stabilized light source in which only a simple, compact and low-cost setup is necessary. A further object of the present invention is to provide a frequency stabilization method of a semiconductor laser and a frequency-stabilized light source in which an output light power and an oscillation frequency of the semiconductor laser can be maintained even if the laser operates for a long period of time. Another object of the present invention is to provide a frequency stabilization method of a semiconductor laser and a frequency-stabilized light source in which unmodulated light output can be obtained. Still another object of the present invention is to provide a frequency stabilization method of a semiconductor laser and a frequency-stabilized light source in which both of the oscillation frequency and the output light power of the laser can be stabilized. Still another object of the present invention is to provide a frequency-stabilized semiconductor laser module which enables the semiconductor laser to operate stably. According to a first aspect of the present invention, a frequency stabilization method of a semiconductor laser is provided, which includes the steps of detecting an output light power of a semiconductor laser mounted on the heat sink, obtaining a consumption power of the laser, obtaining a relationship between the output light power and the consumption power, detecting a temperature of the heat sink, controlling a driving current for the laser so that the output light power is kept constant, and controlling the temperature of the heat sink based on the relationship so that a temperature of an active layer of the laser is maintained. With the frequency stabilization method of the semiconductor laser according to the first aspect, the output light power is kept constant and at the same time, any temperature change of the active layer is cancelled through the temperature control of the heat sink. Therefore, even if the consumption power changes due to an leakage current and/or a recombination current without luminescence in order to maintain the output light power during long time operation, the temperature of the active layer is maintained by cancelling the consumption power change through the temperature control of the heat sink. As a result, the oscillation frequency of the semiconductor laser can be stabilized at a given value for a long time operation, which means no oscillation frequency drift. Additionally, the output light power also can be maintained. Further, since no gas cell and no synchronous detection means are required, only a simple, compact and low-cost setup is sufficient for the method. According to a second aspect of the present invention, a frequency-stabilized light source is provided, which enables the method of the first aspect to be carried out. The light source includes a current detector for detecting a light power detector for detecting an output light power of a laser mounted on a heat sink, a light power detector for detecting an output light power of a semiconductor laser mounted on a heat sink, a first operator for obtaining a consumption power of the laser, a second operator for obtaining a relationship between the output light power and the consumption power, a temperature detector for detecting a temperature of the heat sink, a first controller for controlling a driving current of said semiconductor laser so that the output light power is kept constant, and a second controller for controlling the temperature of the heat sink based on the relationship so that a temperature of an active layer of said laser is maintained. With the frequency-stabilized light source according to the second aspect, the frequency stabilization method of the first aspect is carried out, so that the same effects or advantages as those in the first aspect can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a functional block diagram showing a conventional frequency stabilization method of a semiconductor laser. Fig. 2 is a functional block diagram showing another conventional frequency stabilization method of a semiconductor laser. Fig. 3 is a functional block diagram of a frequency-stabilized light source according to a first embodiment of the present invention. Fig. 3A schematically shows a cross section of a semiconductor laser used in the light source shown in Fig. 3. Fig. 4 is a functional block diagram of a frequency-stabilized light source according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below while referring to the drawings attached. [First Embodiment] In Fig. 3, which shows a frequency-stabilized light source according to a first embodiment, a semiconductor laser 101 is mounted on a heat sink 103 through a subcarrier 103a. A photodetector 115 fixed on the heat sink 103 receives an output light beam of the laser 101 to detect its power and outputs an electrical signal corresponding to the detected power P to a first subtractor 111. The first subtractor 111 produces by subtraction a difference between the detected power P and the predetermined reference power P 0 and outputs an electrical signal corresponding to the power difference thus produced to a current controller 102. In response to this signal, the current controller 102 controls to increase or decrease a driving current for the laser 101 in order to keep or stabilize the power of the output light beam at P 0 . A current detector 113 is arranged in a current path between the current controller 102 and the laser 101 to detect the driving current. The current detector 113 outputs an electric signal corresponding to the driving current thus detected to a temperature controller 106. Thus, the driving current is always monitored by the temperature controller 106 during operation. A voltage detector 114 is electrically connected between input terminals of the laser 101 to detect the voltage therebetween. The voltage detector 114 outputs an electric signal corresponding to the inter-terminal voltage thus detected to the temperature controller 106. Thus, the forward voltage of the laser 101 is always monitored by the temperature controller during operation. The temperature controller 106 carries out a mathematical operation to get the consumption power of the laser 101 based on the output signals from the current and voltage detectors 113 and 114. Then, the controller 106 increases or decreases the reference temperature T 0 of the heat sink 103 based on the result of the operation and sends an electric signal corresponding to the reference temperature T 0 to a second subtractor 112. A temperature detector 104 fixed on the heat sink 103 detects the temperature of the heat sink 103. The detector 104 produces an electric signal corresponding to the detected temperature T to send the signal to the second subtractor 112. The second subtractor 112 produces by subtraction a difference between the detected temperature T and the reference temperature T 0 and outputs an electrical signal corresponding to the temperature difference thus produced to a heat sink controller 105. In response to this signal, the heat sink controller 105 increases or decreases the temperature of the heat sink 103 in order to keep or stabilize the temperature at T 0 . The temperature control by the heat sink controller 105 is carried out using Peltier elements 110 attached to the heat sink 103. Since the Peltier elements 110 generate or absorb heat depending upon its driving current, the temperature of the heat sink 103 can be increased or decreased precisely by the elements 110. Fig. 3A shows a cross section of the semiconductor laser 101 used in the light source shown in Fig. 3. In Fig. 3A, on the surface of a semiconductor substrate 101a, a strip portion formed of an active layer 101c and upper and lower cladding layers 101d and 101b respectively disposed on upper and lower sides of the active layer 101c. The strip portion is bounded on its each side by a pair of burying layers 101e formed on the surface of the substrate 101a. A pair of insulator layers 101f are formed on the surfaces of the pair of the burying layers 101e, respectively. An upper electrode or terminal 101g is formed on the surfaces of the insulator layers 101f and the upper cladding layer 101d. A lower electrode or terminal 101h is formed on the back surface of the substrate 101a. The laser 101 is fixed to the subcarrier 103a through the lower electrode 101h. The pair of the burying layers 101e act as current blocking layers for narrowing the driving current path. The current detector 113 is electrically connected to the upper electrode 101g and the voltage detector 114 is electrically connected to the upper and lower electrodes 101g and 101h. Next, the frequency stabilization operation of the semiconductor laser 101 is described below. The semiconductor laser 101 tends to show a drift or deviation in oscillation frequency in a long period of time when the driving current is controlled so that the output light power of the laser 101 is kept constant and at the same time, the controlling current for the Peltier elements 110 is controlled so that the temperature of the heat sink 103 is kept constant. It is considered that the drift or deviation in oscillation frequency is caused by change in percentage which contributes the laser oscillation within the total driving current injected into the laser 101. The change in percentage contributing laser oscillation is considered to be caused by (a) increase in leakage current due to deterioration of the current blocking layers or the pair of the burying layers 101e, and by (b) increase in recombination current without luminescence due to deterioration of the active layer 101c. For example, when both or the output light power and the temperature are kept constant, the driving current percentage contributing laser oscillation tends to decrease, and the driving current increases to compensate the percentage decrease. Thus, the temperature or the active layer 101c rises in response to the increase in the driving current and as a result, the oscillation frequency of the laser 101 will drift toward the lower frequency side. The drift Δf of the oscillation frequency can be expressed as follows: It is hardly considered that because an optical loss within the semiconductor laser 101 changes during operation in the long period of time, the carrier density in the laser 101 is kept substantially constant during operation and as a result, the oscillation frequency is not affected by the optical loss. Therefore, it is considered that the frequency change or drift Δf is caused by the temperature change ΔT of the active layer 101c. Under the first order approximation, the frequency change Δf can be expressed as Δ f = ∂ f ∂ T ·Δ T The equation (2) means that the frequency change Δf is zero if the temperature of the active layer 101c is constant, that is, the temperature difference ΔT is zero. The temperature of the semiconductor laser 101 can be controlled to be kept constant by the heat sink controller 105, however, there arises a temperature difference between the active layer 101c and the heat sink 103 because of heat resistance between the active layer 101c and the temperature detector 104 when the consumption power of the laser 101 changes. The temperature change ΔT of the active layer 101c is expressed as Δ T = ·Δ W + Δ T 0 where ΔW is the consumption power change of the active layer 101c,  is the heat resistance between the active layer 101c and the temperature detector 104 and ΔT 0 is the change of the reference temperature of the heat sink 103. Consequently, it is seen that the temperature change ΔT 0 can be cancelled if the following equation (4) is established, resulting in a stabilized oscillation frequency. ΔT 0 = --ΔW Here, the total driving current i t and the forward voltage v between the input electrodes 101g and 101h can be expressed as the following equations (5a) and (5b) using the initial, total driving current i t0 , the initial forward voltage v 0 , the change Δi t of the total driving current i t , and the change Δv of the forward voltage. i t = i t0 + Δ i t V = V 0 + ΔV From the equations (5a) and (5b), the consumption power change ΔW of the active layer 101c can be expressed as ΔW = W - W 0 = i t · v - i t0 · v 0 = Δ i t - v 0 + it 0 ·Δ v + Δ i t · Δ v The reference temperature T 0 of the heat sink 103 is controlled by the heat sink controller 105 so that the consumption power change ΔW expressed by the equation (6) is cancelled. The temperature controller 106 calculates in value the consumption power change ΔW using the equations (4) and (6) based on the electric signals from the currents and voltage detectors 113 and 114. Then, the controller 106 adjusts in value the reference temperature T 0 to cancel the calculated value of the consumption power change ΔW. As described above, with the light source according to the first embodiment, the current controller 102 controls the driving current for the semiconductor laser 101 so that the output light power of the laser 101 is kept constant, and at the same time, the temperature controller 106 and the heat sink controller 105 control the reference temperature T 0 of the heat sink 103 so that the temperature change of the active layer 101c is cancelled. As a result, the oscillation frequency of the laser 101 can be stabilized because of no oscillation frequency drift due to electrical and optical characteristic changes of the laser 101 with passage of time. Additionally, no gas cell and no synchronous detection means such as a lock-in amplifier are required, so that there arises an advantage that a simple, compact and low-cost setup is sufficient for the light source. [Second embodiment] Fig.4 shows a frequency-stabilized light source according to a second embodiment of the present invention. In Fig. 4, the same reference numerals as those in Fig. 3 are attached to the corresponding elements for the sake of simplification of description and illustration. A current control circuit 216 for controlling the driving current I for the semiconductor laser 101, a temperature control circuit 217 for controlling the temperature of the heat sink 103, and a micro computer 21 for controlling the circuits 216 and 217 are provided instead of the current controller 102, the temperature controller 106 and the heat sink controller 105 in Fig. 3. The temperature detector 104 sends an electric signal corresponding to the detected temperature T of the heat sink 103 to the micro computer 218. The photodetector 115 sends an electric signal corresponding to the detected light power P of the laser 101 to the micro computer 218. The voltage detector 114 sends an electric signal corresponding to the detected voltage V of the laser 101 to the micro computer 218. The micro computer 218 is storing data relating the reference power P 0 and the reference temperature T 0 in advance. The computer 218 calculates data using the signal about the output light power P from the photodetector 115 and sends the data to the current control circuit 216. In response to the data thus sent, the current control circuit 216 controls to supply the driving current to the semiconductor laser 101, so that the output light power from the laser 101 is kept constant. On the other hand, the computer 218 calculates data using the signals about the forward voltage V from the voltage detector 114 and the temperature T from the temperature detector 104, and sends the data to the temperature control circuit 217. In response to the data thus sent, the temperature control circuit 217 controls to supply the driving current to the Peltier elements 110, so that the reference temperature T 0 of the heat sink 103 is adjusted in order to cancel the temperature change of the active layer 101c. Therefore, in the second embodiment, the same effects as those the first embodiment can be obtained. Additionally, there is an advantage that the configuration or setup can be made simpler than that of the first embodiment because of the micro computer 218.

Claims: A frequency stabilization method of a semiconductor laser, said method comprising the steps of: detecting an output light power (P) of a semiconductor laser (101) mounted on a heat sink (103); obtaining a consumption power of said laser; obtaining a relationship between said output light power and said consumption power; detecting a temperature (T) of said heat sink; controlling a driving current (T) for the laser so that said output light power is kept constant; and controlling said temperature of said heat sink based on said relationship so that a temperature of an active layer of said laser is maintained. A frequency stabilization method of a semiconductor laser as claimed in claim 1, further comprising the steps of: detecting said driving current of said semiconductor laser; and detecting a voltage drop (V) of said laser; wherein said relationship between said output light power and said consumption power is obtained based on detection results of said driving current and said voltage. A frequency-stabilized light source, comprising: a light power detector (115) for detecting an output light power (P) of a semiconductor laser (101) mounted on a heat sink (103); a first operator for obtaining a consumption power of said laser; a second operator for obtaining a relationship between said output light power and said consumption power; a temperature detector (104) for detecting a temperature (T) of said heat sink; a first controller (102) for controlling a driving current (I) of said semiconductor laser so that said output light power is kept constant; and a second controller (106) for controlling said temperature of said heat sink based on said relationship so that a temperature of an active layer of said laser is maintained. A frequency-stabilized light source as claimed in claim 3, further comprising: a current detector (113) for detecting said driving current of said semiconductor laser; and a voltage detector (114) for detecting a voltage drop (V) of said laser; wherein said relationship between said output light power and said consumption power is obtained based on detection results of said driving current and said voltage.

Inventors: HENMI NAOYA, SHIOZAWA TAKAHIRO, YAMAGUCHI MASAYUKI, HENMI, NAOYA, SHIOZAWA, TAKAHIRO, YAMAGUCHI, MASAYUKI, HENMI, NAOYA, C/O NEC CORPORATION, SHIOZAWA, TAKAHIRO, C/O NEC CORPORATION, YAMAGUCHI, MASAYUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S5